1. Field of the Invention
The present invention relates particularly to a heat conductive composite sheet that is ideally suited to use as a heat radiating structure provided between a heat generating electronic component or the like and a heat radiating component such as a heat sink or a circuit board for the purposes of radiating heat away from the heat generating electronic component or the like. The invention also relates to a process for producing this type of composite sheet.
2. Description of the Prior Art
In the circuit design of recent electronic equipment such as televisions, radios, computers, medical instruments, office equipment and communication devices, advances in miniaturization of these devices has led to great increases in complexity in the circuit design. Integrated circuits are now being produced for electronic equipment incorporating hundreds of thousands of transistors. The complexity of the circuit designs continues to increase, while the size of the electronic components continues to shrink, making it possible to incorporate more and more components into even smaller areas, and reducing the dimensions of devices even further.
These electronic components, particularly IC packages such as CPUs and the like that are mounted onto printed circuit boards, suffer from deterioration in performance, faults and other malfunctions as the temperature rises under the influence of heat generated during use of the component. In order to alleviate these problems associated with heat generation, conventionally a heat conductive grease or silicone rubber sheet or the like is disposed between the IC package and a heat sink. However, as the size of electronic components has decreased and the performance has increased, the quantity of heat generated by the components has also increased, meaning the development of members with even better heat radiating properties is being keenly pursued.
An advantage of conventional heat conductive greases is their low interface thermal resistance, meaning they are able to closely contact and follow the surface to which they are applied, and are unaffected by any irregularities in the surface of the electronic component or the heat sink. However such greases also have a number of drawbacks including soiling of other components, bleeding or separation of oil after extended use, and an increase in viscosity over time that leads to the grease bonding too strongly to the coated surface. This strong bonding means that during rework, for example when the heat conductive member must be removed temporarily to enable the electronic component to be repaired or replaced, the electronic component such as a CPU tends to be removed together with the heat conductive member.
Heat conductive silicone rubber sheets offer different advantages in that they are easy to mount, and display good reworkability. However, these sheets also have drawbacks, including an upper limit to the quantity of heat conductive filler that can be incorporated within the rubber sheet, in order to maintain sufficient workability during the production process, increased likelihood of unsatisfactory ability of the sheet to closely contact and follow the surface to which it is applied, and inadequate heat radiating performance due to a large interface thermal resistance when the sheet is mounted.
As a possible solution to the above problems, heat radiating sheets comprising a layer with heat softening characteristics have been disclosed in Published Japanese translations of PCT international publication (kohyo) No. 2000-509209 (JP2000-509209A) and Japanese Laid-open publication (kokai) No. 2000-327917 (JP2000-327917A). However with these sheets, although the heat softened resin component follows and closely contacts the surface to which it is applied, leaving no gaps and preventing any increase in interface thermal resistance, problems still remain. Namely, there is still an upper limit to the quantity of heat conductive filler that can be incorporated within the sheet, in a similar manner to conventional rubber sheets, the resin itself can sometimes become a cause of interface thermal resistance, and in some cases the sheet may bond too strongly to the coated surface after extended use at high temperature, in a similar manner to the heat conductive greases described above, resulting in a deterioration in reworkability.